Edit - Fill

Open a picture that is on part of a page eg empty background due to using vector mask layer.
To add a background - select the layer with the vector mask and the picture - select new layer from the bottom right - second last button
Select the new layer with the mouse button and drag it below the layer with the picture. Select the new layer - edit - fill
This opens a menu = we need to fill the layer with a colour - ok - ok
Cropping tool - crop to size - enter
Double click on hand
This makes a comp (composition)